Field of the Invention
The present invention concerns a method for rephasing a first spin system in a first slice with a first coherence curve and a second spin system of a second slice with a second coherence curve in the generation of magnetic resonance (MR) images, and a magnetic resonance system designed to implement such a method. In particular, the invention concerns the rephasing of spin systems in slice multiplexing measurement sequences.
Description of the Prior Art
Magnetic resonance tomography is an imaging method that is used for examination and diagnosis in many fields of medicine. The phenomenon of nuclear magnetic resonance forms the basis of this technique. To acquire MR signals, a static basic magnetic field is generated in the examination region at which the nuclear spins (magnetic moments) of the atoms in the examination subject align. The nuclear spins can be deflected or excited out of the aligned position (i.e. the idle state) or a different state by radiating the radio-frequency pulses. The relaxation back into the idle state generates a decay signal that can be inductively detected by one or more reception coils.
The phase evolution of the spin system of a slice is described by the coherence curve (progression). If the spins of a spin system of a specific slice all have an identical phase position, this is known as a disappearing dephasing of the coherence curve. A signal can be detected since no destructive interference exists between the signals of various spins of different phase.
By applying a slice selection gradient upon radiation of the radio-frequency pulses, nuclear spins are excited only in a slice of the examination subject in which the resonance condition due to the local magnetic field strength is satisfied. An additional spatial coding can take place by the application of at least one phase coding gradient, and a frequency coding gradient can be activated during the readout. It is thereby possible to acquire MR exposures of multiple slices of an examined person. With suitable presentation methods, it is possible to provide a 3-dimensional (3D) image of a specific region of the examined person for diagnosis.
In the clinical environment there is always a quest for faster MR acquisitions, in particular 3D acquisitions. MR measurement sequences to generate MR exposures can be optimized in this regard. In particular, MR sequences in which images are acquired simultaneously from multiple slices, i.e. what are known as slice multiplexing measurement sequences, are suitable for this purpose. In general, slice multiplexing measurement sequences can be characterized by a transverse component of the magnetization of at least two slices being specifically used simultaneously for the imaging process, at least during a portion of the measurement. By contrast, in the usual multislice imaging, the signal of at least two slices is acquired in alternation, i.e. completely independently of one another and with a correspondingly longer measurement time (known as “interleaved” measurement sequences).
Various slice multiplexing measurement sequences are known. For example, given simultaneous excitation of the magnetization and/or simultaneous detection of an MR signal the addressing of the various slices can take place via a phase coding (what is known as “Hadamard” coding; see S. P. Souza et al. in J. Comput. Assist Tomogr. 12 (1988) 1026) or a frequency coding (what is known as “broadband data acquisition”; see E. L. Wu et al. in Proc. Intl. Soc. Mag. Reson. Med. 17 (2009) 2678).
Furthermore, there are MR measurement sequences that use multiple radio-frequency coils to differentiate various slices. With knowledge of the spatial acquisition characteristic of the different radio-frequency coils, the simultaneously acquired data can be separated by means of suitable computing operations. Such methods are known under the names SENSE, GRAPPA or SMASH, for example.
In the simultaneous application of multiple radio-frequency pulses to deflect the magnetization (meaning that the magnetization of various slices is simultaneously affected by means of radio-frequency pulses), although the time required to implement the measurement sequence is shortened, the required peak power of the radio-frequency alternating electromagnetic field is simultaneously increased. This increases the specific absorption rate (SAR) in the examined person and is generally not desired. In this context U.S. Pat. No. 5,422,572 discloses a method of parallel MR imaging in which various slice-selective radio-frequency (RF) pulses are essentially applied simultaneously, but with a slight time shift relative to one another. Both the measurement time and the required peak RF power can thereby be reduced.
It should be noted that the time shift of the RF pulses results in differences in the temporal evolution of the magnetization (coherence curve). In order to ensure a simultaneous data acquisition, the coherence curve of the spin systems of the various slices must be rephased at the point in time of the data acquisition.
In simultaneous acquisition of MR data from multiple slices, it is accordingly desirable to rephase the coherence curves of the corresponding spin systems at the point in time of the data acquisition.